The pace at which Americans signed up for health plans slowed last month in the fledgling federal and state insurance marketplaces, according to new government figures showing that slightly fewer than 1 million people enrolled in February.

The Obama administration said 943,000 Americans selected health plans, compared to 1.2 million in January. Overall, enrollment stood at 4.2 million as of the end of last month.

Contrary to the Obama administration’s expectations, fewer people chose health plans last month than in either January or December. And the proportion of young adults — a critical demographic if the marketplaces are to function well — did not increase compared with January.

The enrollment tallies suggest that administration officials and their allies, who have undertaken a blitz of outreach activities, face a significant challenge in reaching their enrollment goals by March 31, the end of the initial six-month sign-up period.

In a conference call with reporters, administration officials said the smaller number of enrollments reflects the fact that February is a short month. They predicted a surge of sign-ups in the remaining few weeks that Americans have to choose coverage for 2014.

“We do believe millions more Americans will come in and enroll in coverage before the March 31 deadline,” said Julie Bataille, spokeswoman for the Department of Health and Human Services. “We’ve seen that many consumers simply choose to make this kind of decision very close to the deadline.”

Congressional budget analysts last month projected that 6 million people would sign up by the end of March — but the new report raises doubts about whether that will be achieved.

Polls show the country is still deeply divided over the law, and that the uninsured in particular are skeptical that they can afford new health plans. Some experts have warned that a new fine for failing to carry health insurance — as low as $95 in 2014 — may be too low spur people to buy coverage.

The report comes as President Obama steps up his promotion of the health-care law. He made a targeted pitch to young adults Tuesday with an appearance on the online series “Between Two Ferns,” hosted by Zach Galifianakis, who starred in the “Hangover” films.

Strong participation by young adults is critical to the program’s success, because they tend to use less medical care. Because they are cheaper to ensure, young people offset insurers’ costs of covering the sick, many of whom are eager to sign up for coverage. Under the health-care law, people with preexisting medical conditions can’t be rejected.

Initially, officials had hoped that 40 percent of the sign-ups would be adults under the age of 35, but only about 27 percent of February enrollments were young adults, about the same as in January. On Tuesday, administration officials said they were nevertheless encouraged and predicted more young people would enroll closer to the deadline.

That view was echoed by Enroll America, a national nonprofit formed to enroll people in coverage under the law. “We know from our research and past enrollment efforts that many people, especially young folks, will wait towards the end to sign up, which is why we’ve ramped up our outreach efforts during this final push,” president Anne Filipic said in a statement.

But the numbers drew an immediate rebuke from Republicans, who renewed their calls to delay the fine to be imposed on people who fail to carry coverage.

“It seems the president’s push to enroll young adults is far too little, too late,” Brendan Buck, spokesman for House Speaker John A. Boehner (R-Ohio) said in a statement. “Given these dismal enrollment numbers, the president needs to work with Congress to get rid of this year’s individual mandate penalty.”

Last year, administration officials embraced forecasts by congressional budget analysts that 7 million Americans would get coverage through the federal and state insurance marketplaces by this spring. Early last month, those analysts lowered their forecast to 6 million, reasoning that enrollment would be depressed somewhat by technical problems involving HealthCare.gov and a few state-run marketplaces.

Enrollment in the marketplaces was sluggish through most of the fall but accelerated during the winter. January marked the first time that the number of people who signed up on HealthCare.gov or 14 separate state-run marketplaces was higher than what the adminstration predicted last summer.

Those projections, contained in a memo by Marilyn Tavenner, administrator of the Centers for Medicare and Medicaid Services, the agency overseeing the marketplaces, anticipated that nearly 1.3 million people would select a health plan during the month of February and that the overall enrollment by then would slightly exceed 5.6 million.

The new progress report, like the four that preceded it, contains a major limitation. It shows the number of Americans who have chosen a health plan via the marketplaces, but it does not show the number who have actually started to pay premiums — a figure the government has not disclosed so far.

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